This invention relates to the use of viruses as carriers (vectors) for the production or presentation of foreign peptides. More particularly, the invention relates to the genetic manipulation of viral nucleic acid by incorporation of foreign nucleic acid sequences which are expressed as peptides in the virus particle (virion). In this specification the term "foreign", as applied to a peptide or to the nucleic acid encoding therefor, signifies peptide or nucleic acid sequences which are not native to the plant virus used as a vector. Such sequences can be alternatively described as exogenous or heterologous sequences. The term "peptide" includes small peptides and polypeptides.
The use of viruses as carriers of foreign peptides has been extensively explored in the field of composite virus vaccines. Such vaccines are based on chimeric viruses which are hybrids of different animal virus components. Usually the major component of such hybrids is derived from a virus which is or has been rendered harmless and the minor component is a selected antigenic component of a pathogenic virus. For example, a pox virus such as vaccinia or an attenuated poliovirus may be used as a vector for immunogenic components of other animal viruses including human viruses.
However, the above technique has several disadvantages. Such vaccines are produced from viruses grown in cell culture systems which are expensive to design and run. The composite virus approach involves genetic manipulation of live, animal-infecting viruses, with the risk that mutations may give rise to novel forms of the virus with altered infectivity, antigenicity and/or pathogenicity. The animal virus used as the vector is often a virus to which the animal may already have been exposed, and the animal may already be producing antibodies to the vector. The vector may therefore be destroyed by the immune system before the incorporated antigenic site of the second virus induces an immune response.